1. Field of the Invention
This invention relates to a semiconductor memory cell which is capable of easily forming a read-only memory (ROM) cell from a random access memory (RAM) cell.
2. Description of the Prior Art
A gate array method as a design method or a development method for various kinds of custom LSIs is widely used because this gate array method has a short development period.
In the gate array method, a plurality of basic cells are arranged and secured to a chip. These basic cells are used for designing various kinds of random logic LSIs to reduce the development period.
In the gate array method, there are two methods of designing memory circuits which are commonly used.
In the first method, the basic cells are wired by using mask patterns (which have been registered in a library) to form various types of memory cells. The RAM and ROM cells can be formed as memory cells with different configurations because the memory cells used in the first method are a general purpose type memory cell.
However, the required areas of the RAM and ROM on the chip are increased because the configurations of the RAM and the ROM cells are wasteful and the wiring required in the RAM and ROM is relatively long.
Therefore, it is difficult to design an LSI by using the first method.
In the second method, special memory cells for various memory circuits are registered in the library, and a memory cell is designed by using the special memory cells.
By using the second method, the problems inherent in the first method (wasted area in the RAM and ROM cells) can be eliminated. However, the special memory cells such as the RAM cells and the ROM cells must be registered because the functions of the RAM and ROM cells are different.
Moreover, differrent capacity memory cells must be registered for the RAM cells and the ROM cells in the sercond method. There are, therefore, more kinds of memory cells than in the first method.
When a custom LSI incorporating memory circuits is designed by using the second method, special memory cells for required functions must be developed or the memory cells having such functions must be registered beforehand in a library, so that the design cost is increased.